1. Field of the Invention
This invention relates generally to chairs for use in offices for example. More particularly, the invention relates to chairs of the type wherein the chair seat and/or the chair back are designed to be tiltable at least rearwardly downward against a spring or springs.
2. Description of the Prior Art
There have been proposed various types of tiltable chairs wherein at least one of the chair seat and the chair back is tiltable against a tilting control spring or springs. The most typical is a rocking chair wherein the seat is rearwardly tiltable together with the chair back. Such a chair enables the user to assume a relaxing posture occasionally during desk work for example.
As is well known, the tilting degree or angle of a tiltable chair seat and/or back is generally proportional to the weight of the use but inversely proportional to the spring constant of a tilting control spring or springs. Thus, for a given weight, the tilting degree increases with decreasing spring constant, and decreases with increasing spring constant.
Most commonly used as a tilting control spring is a coil spring whose spring constant is invariable. Thus, a tiltable chair utilizing a tilting control coil spring or springs has a disadvantage that the tilting degree inevitably varies depending on the weight of a particular user with no possibility of adjusting the tilting degree to suit the user's weight.
U.S. Pat. No. 4,077,596 discloses a chair tilting control assembly which comprise a pair of tilting control plate springs each fixed at one end in a cantilever fashion for elastically allowing rearwardly downward tilting of the chair seat and back. Specifically, the weight of the sitter applied to the seat is elastically supported by the plate springs via a U-shaped rod which provides a load applying member carried by the seat. The U-shaped rod is designed to be advanced or retreated relative to the plate springs by manually turning an adjusting screw. Thus, the spring constant of the plate springs can be adjusted to suit the weight of the user.
However, the tilting control assembly of the above U.S. patent is disadvantageous in that it requires manual adjustment upon every change of the user. Further, the manual adjustment is cumbersome and time-taking, so that the user often prefers uncomfortable chair tilting than making such an adjustment. Moreover, the manual adjustment is a guess game, and therefore does not necessarily result in comfortable chair tilting.
Further, each tilting control plate spring of the above U.S. patent is made to extend away from the pivotal axis of the seat, and the U-shaped rod as a load applying member is advanced closer to the pivotal axis (toward the fixed ends of the springs) for increasing the spring constant of the tilting control spring. Therefore, the reaction moment arm length of the tilting control spring decreases as the spring constant thereof increases. Obviously, with a larger reaction moment arm length, the plate spring provides a stronger support against tilting of the chair. Thus, the adjustment of the spring constant is contradicted by simultaneous reduction of the reaction moment arm length.
Moreover, each tilting control spring of the above U.S. patent is fixed in position, and has a uniform width and thickness. Thus, the tilting control spring has a limited range of spring constant adjustment, and is therefore incapable of providing an optimum tilting support for a wide range of users varying in weight for example from 40 kg to 100 kg.